Shift debian element to DIB_DISTRIBUTION_MIRROR

The ubuntu element supports simple replacement of sources.list by using
the DIB_DISTRIBUTION_MIRROR environment variable. Drag the debian
element into line by supporting both DIB_DEBIAN_MIRROR and the new shiny
DIB_DISTRIBUTION_MIRROR.

Change-Id: I9d5f8d1e3251965b34e55929182aa601a524fe8f
Merge "Shift debian element to DIB_DISTRIBUTION_MIRROR"